This invention relates to the use of involatile alcohols or polyols as components of water-in-oil emulsion inks, which contain pigment and resin in the water phase. The purpose of this invention is to achieve an improved ink for use on digital duplicators, which provides optimum image recovery performance.
The printing performance of digital duplicators has been subject to a series of improvements since their introduction. Recent developments in the design of the printing equipment and stencil master have enabled full rendition of the printed image from the first print in a printing run. Such performance could not be achieved with previous designs because, at the start of printing, ink would not have fully penetrated the porous backing layer of the digital stencil. This porous layer acts as an ink reservoir during the printing process. When there is insufficient ink present within this reservoir the initial prints are incomplete or patchy in appearance and full copy quality is only achieved when an adequate volume of ink is present within the porous layer.
Clearly it is desirable that the achievement of good copy quality from the first print is a consistent aspect of the performance of the digital duplicator. However it is known that when digital duplicators are allowed to remain unused for a period of time, for example in an office or school where no printing takes place over the period of the weekend, then it is frequently necessary to run a number of prints before full copy quality is achieved. This process of regaining full copy quality is termed “image recovery” and the number of copies from the start of printing to achieve this result is used as a quantitative measure of performance. It is an objective of this invention to achieve inks that give the lowest possible values for image recovery even after a prolonged period of non-use. Typically the inks used for this type of printing process are water-in-oil emulsions and the main reason for an increase in image recovery values after a digital duplicator has remained unused for a period of time is a change in the properties of the ink within the printing drum as a result of the evaporation of some or all of its water content. These changes are such that it takes a longer period of time for the ink to penetrate through the screens on the printing drum and through the porous layer of the stencil master.
U.S. Pat. No. 3,421,910 discloses water-in-oil emulsion inks for stencil duplication said to have good copy quality when used on a rotary stencil duplicating machine. One of the exemplified inks comprises a pigment and glycerol in the water phase, and rosin in the oil phase. U.S. Pat. No. 3,421,910 discloses that polyols other than glycerol may be incorporated into its inks, such as diethylene glycol and ethylene glycol. No mention is made of the image recovery properties of the disclosed inks.
U.S. Pat. No. 3,823,020 makes reference to U.S. Pat. No. 3,421,910 and discloses that an unexpected improvement in ink stability can be achieved by formulating inks without a polyol.
EP-A-0661356 and EP-A-0694595 also disclose water-in-oil emulsion inks comprising pigment and resin in the water phase, together with the use of the relatively volatile alcohol, ethylene glycol as a wetting agent. Inks of this type are capable of providing excellent print quality in terms of set-off, and fixing of print to paper. Also, in conjunction with an appropriate design of digital duplicator and stencil master, it is possible to achieve very low values for immediate image recovery. Immediate image recovery is a measure of the number of prints required to achieve full copy quality when the ink is immediately installed on a digital duplicator. However, after leaving a digital duplicator to stand with this type of ink installed for a period such as a weekend, a severe deterioration in image recovery performance is experienced.